I had a thought error when reading Mollier diagrams and thankfully we have the internet to assist, but even after finding the correct sources showing the answers, I can't get out of my thought-loop to understand why my thinking is wrong. I hope someone could in simple and very definite terms explain why my thinking is wrong.

So let's say that I have the dry bulb temperature of 20 Celsius and wet bulb temperature of 15 Celsius and I'd like to know the relative humidity.

enter image description here

My thought error was that I thought that I'd read this from the 100% humidity point at 15 Celsius, straight up along the absolute humidity lines. My thought here was that the absolute humidity is the same and the air's water content doesn't change no matter how much I spin my wet-bulb thermometer. So I'd have assumed from the image for the relative humidity to be about ~72% at 20 C. (red arrow)

However this source showed how it should be read and I was also able to test it online. So I should actually read it along the enthalpy lines, which gives me about 58% relative humidity at 20 C. (yellow arrow)



Now I'm a bit stuck: I can from one perspective understand that the enthalpy does not change and is constant, but I'm having a hard time to definitely understand why my initial assumption about the absolute humidity is the "wrong constant" in this case.

Could someone take it down to my level, and also possibly clarify if there is something I still seem to have misunderstood, or what should be good to recognize?


1 Answer 1


Your thinking is correct (red line) if you would use dew point temperature (same absolute humidity but at saturation point = 100% relative humidity line. It is different air state, with different enthalpy) and dry air temperature. However as you wish to define your point using wet bulb temperature (enthalpy of air does not change regardless how you measure it - using dry air thermometer or wet bulb thermometer) and dry air temperature, the yellow line is correct.

  • $\begingroup$ I would only like to add that the diagram shows h-lines and not wet bulb lines; they are not completely same, but very very close, so this little distinction does not significantly influence the result. $\endgroup$
    – Pygmalion
    Commented Jul 16, 2023 at 20:05

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